A variety of methods and systems exist for tracking and locating assets and personnel in a facility such as a hospital or a factory. Present tracking methods currently available fall into several broad categories of operation: 1) control point; 2) time of arrival; and 3) GPS.
In control point schemes asset tracking tags are affixed to assets to be tracked. One such control point approach is presented in U.S. Pat. No. 6,344,794. Other control point methods are based on RFID technology similar to the system marketed by AXCESS Inc. The tracking tags can be either passive (e.g., deriving power from an external source) or active (e.g., deriving power from an internal source). The tracking tags emit a signal either upon request, periodically or sporadically. The signal can be a light signal, infra-red or RF signal and is modulated with an identification number unique to the asset to which the tracking tag is affixed.
Control point schemes require that a multitude of control points be established. Control points are usually established at doorways or in the geometric center of rooms of interest. At each control point a receiver, reader or access point is placed. The control points are typically tied together by a communications network which may include a central processor and database. When the tracking tag is in close proximity to a control point, the control point receives an identification number signal from the tracking tag.
There are several limitations to control point systems as they relate to asset tracking. The ability of the control point system to resolve the precise location of a tracking tag is limited. The control point system can only report that a given tracking tag is either within range or not within range of a given control point. In addition, there are large gaps in coverage between control points due to the propagation characteristics of IR and RF signal energy.
Time of arrival tracking systems determine the location of an asset by measuring the time it takes for radio signals to travel from multiple fixed radio antennas to the asset tracking tag. Conversely, a time of arrival system can be established by measuring the time it takes the signal to travel from the asset tracking tag to multiple fixed antennas. In some systems the multiple fixed antennas of time of arrival systems are installed specifically for the purpose of making time of arrival measurements. In still other systems time of arrival calculations are performed on signals emanating from antenna infrastructures that are already in place for other purposes such as wireless computer network access points or FM radio broadcast stations. One such time of arrival approach is presented in U.S. Pat. No. 4,799,062.
There are several limitations to time of arrival systems as they relate to asset tracking. The ability of time of arrival systems to resolve location requires an accurate time reference be available throughout the system. Each transmitter and receiver must be synchronized, either by direct or indirect means, to a known reference in order for time of arrival to be measured. Another set of limitations arises from the reflective nature of radio wave propagation. Multipath reflections can substantially increase the time of arrival for radio signals which will degrade the accuracy of any such system. Multipath reflections are especially problematic in an indoor environment. Another problem with time of arrival systems is that their accuracy is a function of the spatial diversity and number of time-of-arrival paths that can be determined. This limits the useful coverage area of a system to those areas where a transmitting tracking tag is within range of a minimum of three reference receivers. It would be desirable for an asset tracking tag to reliably report its position in the case where only one receiver was in communication range.
A well-known embodiment of a time of arrival system is the GPS satellite system. There are several limitations to the GPS system as it relates to asset tracking. The primary limitation is that GPS signals do not penetrate buildings and therefore assets in places such as hospitals and factories can not be located with GPS.
U.S. patent application Ser. No. 10/432,339 provides an asset tracking system utilizing inertial sensors. Accelerometers and gyroscopes are used to establish an inertial reference from which linear acceleration and roll rates are measured when the asset tracking tag is moved. A limitation with this tracking system is it provides no means of mitigating sensor errors inherent to all inertial sensor techniques such as noise, drift, error in calibration factors, and mechanical alignment errors.
What is needed is a tracking system that may be used indoors and outdoors. Additionally, what is needed is a tracking system that permits precise detection of an asset location.